In order to improve an automobile and a bicycle in riding comfort in response to a road condition, a large number of inventions have been disclosed with respect to a coil spring for supporting a vehicle body. For example, in a coil spring described in both Japanese Utility Model application Laid-open No. Sho 56-20140 and Japanese Patent Publication No. Hei 2-22252, an elastic spacer is mounted in an intermediate portion of a coil element rod of the coil spring in order to change a coil constant in response to a road condition.
In a coil spring described in Japanese Utility Model application Laid-open No. Sho 57-113739, a coiled spacer provided with an appropriate number of convolutions is inserted into a proximal terminal portion of the coil spring, so that the number of effective or active convolutions of the coil spring is changed so as to change the coil spring in its coil constant.
Further, a coil spring described in Japanese Patent application Laid-open No. Hei 6-42563 relates to a wearing washer for preventing the coil spring from inclining due to its buckling when the coil spring is of an open-end type.
In contrast with these coil springs, the present invention improves a coil spring in spring properties by decreasing an amount of initial deflection, wherein the initial deflection is a problem inherent in a coil spring of a closed-end type as described later.
Coil springs used in a buffer spring and like components of an automobile are classified into: a coil spring of closed-end type as shown in FIG. 3(a), wherein a terminal portion of the coil spring is subjected to a flattening process; and, a coil spring of open-end type as shown in FIG. 3(b), wherein a terminal portion of the coil spring is constructed of a part of an active convolution of the coil spring.
In the case of the coil spring of open-end type shown in FIG. 3(b), when a compression force is applied to the coil spring by using a flat washer, an oblique force which is obliquely directed with respect to a coil axis of the coil spring is developed to have the coil spring inclined under the influence of its buckling caused by such oblique force.
Consequently, this type of the coil spring is disadvantageous in that it is necessary for the coil spring to use a special washer such as one disclosed in the Japanese Patent application Laid-open No. Hei 6-42563. In contrast with this, in the case of the coil spring of closed-end type as shown in FIG. 3(a), even when a flat washer is used, it is possible for the flat washer to apply a force uniformly to the coil spring in axial direction thereof without using any special washer. Consequently, in this respect, the coil spring of such closed-end type is advantageous.
In the case where a compression force is applied to the coil spring: when the coil spring is of the open-end type, as shown in dotted line of FIG. 4(a), a plot of the coil spring of the open-end type forms a straight line from its starting point in a load-deflection diagram of FIG. 4(a). As for the spring constant too, as shown in dotted line of FIG. 4(b), a plot of the coil spring of the open-end type forms a straight line from its starting point in a coil spring-deflection diagram of FIG. 4(b). In contrast with this, in the case of the coil spring of closed-end type, as shown in solid line of FIG. 4(a): a terminal portion of the coil element rod of the coil spring is initially deflected under a relatively light load since such terminal portion of the coil spring of the closed-end type is smaller in cross-sectional area than any one of active convolution portions of the same coil spring; and, after that, a plot of the coil spring of the closed-end type reaches its predetermined consistent form in the load-deflection diagram of FIG. 4(a). As for its spring constant, as shown in solid line of FIG. 4(b), a plot of the coil spring of the closed-end type gradually increases from its stating point, and reaches its consistent value at a completion point of its creeping. In this respect, the coil spring of the closed-end type is disadvantageous. In the present invention, such initial deflection occurring in the coil spring of the closed-end type under the relatively light load is hereinafter referred to as “initial deflection”.
In order to avoid the above-mentioned disadvantage inherent in the coil spring of the closed-end type, it is considered that a preliminary load is applied to the coil spring in installation to preliminarily remove the initial deflection of the coil spring from the coil spring's installation height. However, in this method, since the coil spring is substantially decreased in coil height in installation, it is not possible to carry out this method in some application cases. Due to this, a need exists in the art for the coil spring of the closed-end type, which spring is substantially released from a problem of the initial deflection by using a flat washer.